Here is the thing: I don't yet fully understand how the module can help me generate the tileset needed by Tiled to allow me to design my map. In other words, how can I go from the bunch of prefabs I have on a side (more then 200) to a tileset definition (image + coordinates) that I can attach to my tilemap in Tiled? Is this where your Maya script is used? I understand that I have to choose a unit, place my prefabs accordingly (the variant in the Y axe), but how is the tileset image generated with the right size?

If I understand your question correctly, the way the tileset 2D image is generated is up to you. You can draw the 2d tileset as top down representations, or (if you are like me) I like to render them out from a top view orthographic camera in Maya and then in photoshop I crop it at the edges of the tiles and size it down to 1024.

Then, you take that top view image and you can create a tileset and map with it in Tiled. Try making a simple tileset in tiled, like a test one and make a map with it and I think it will all become clear. *Tiled also does come with a simple test one of some pixel desert tiles.